Postnatal development of rat nucleus tractus solitarius neurons: morphological and electrophysiological evidence.

Postnatal development of neurons in the caudal nucleus tractus solitarii of rats was studied using the Golgi-Cox technique and whole-cell recordings. Two cell classes were defined on the basis of somatic and dendritic morphology. Elongated neurons have two thick primary dendrites originating from the long axis of the soma. The primary dendrites, tapering distally, give rise to one to four secondary dendrites. Multipolar neurons have pyramidal somas. Extending from each apex of the cell body was a long primary dendrite, which gave rise to a variable number of secondary dendrites. The relative proportion of the two classes was rather constant from birth to adulthood. During the first two postnatal weeks, dendritic length and area of influence increase, but neuronal geometry is not altered in either class. Dendritic appendages appear by postnatal day 5, reach a peak at postnatal day P12 and then almost disappear in adult neurons. Combined intracellular injection of neurobiotin and whole-cell recordings indicate that morphological alteration of caudal nucleus tractus solitarius neurons occurs in parallel with changes in passive properties and spike characteristics. However, the firing pattern of discharge is not correlated with morphology. The physiological significance of these results is discussed.